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Common Shares (Narrative) (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended
May 05, 2017
Nov. 30, 2014
Mar. 31, 2017
Dec. 31, 2016
Treasury Stock, Shares Repurchased     10,125,000 10,330,000
Treasury Stock, Value of Shares Repurchased     $ 365,923 $ 373,339
Number of common and preferred stock authorized to issue     185,000,000  
Common shares, authorized     175,000,000 175,000,000
Preferred shares, authorized     10,000,000  
Common Stock, Shares, Outstanding     87,600,000  
Preferred Stock, Shares Outstanding     0  
2014 ATM program [Member]        
Maximum aggregate offering price of common shares   $ 331,300    
Common shares issued     0  
2014 ATM program [Member] | Subsequent Event [Member]        
Maximum aggregate offering price of remaining common shares available for sale $ 315,300      
Common shares issued 0      
April 2007 Repurchase Plan [Member]        
Treasury stock allowed for repurchase     $ 500,000  
Treasury Stock, Shares Repurchased     4,300,000  
Treasury Stock, Value of Shares Repurchased     $ 230,200  
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]        
Share Repurchase Program, Remaining Authorized Repurchase Amount $ 269,800